An example of such a power tool from the prior art is shown in EP 0,999,906-A. The tool employed, a rivet gun, uses compressed air to spin its externally-threaded drive screw into a correspondingly internally-threaded blind rivet shank before inserting and then setting the rivet in a workpiece. The force required to achieve the initial part of this process, the so-called “spin-on” of the rivet onto the tool's drive screw is relatively weak, as no structural deformation of the rivet is yet required. All that is needed is for the rivet to be mounted on the drive screw of the rivet gun as quickly and efficiently as possible. This is necessary as, in a manufacturing environment, time required to have the gun ready to set the rivet in a workpiece needs to be minimised for production line efficiencies. Once the rivet has been spun-on to the drive screw, it is ready to be inserted into a hole formed in a workpiece and upset, or deformed. It is this deformation process which requires a relatively higher force than the initial spin-on force. For this deformation, an oil reservoir is employed within the tool to drive a hydraulic ram in order to axially deform the rivet such that it is then permanently mounted within the workpiece. Such axial rivet deformation is, per se, known and so will not be described further herein.
EP 0,999,906-A discloses a rivet gun, rather than a static piece of installation equipment (such as a floor-mounted machine), as it is both manually held and operated. Such manual operation tends to occur in industrial manufacturing environments where use of automated machines is sporadic or expensive or in the case where the operator's manual dexterity is required. However, manual use of the rivet gun brings its own problems. One such problem is the propensity for an operator to drop the rivet gun, possibly damaging the drive screw. Damage could include bending the drive screw out of true, or scraping its external thread. In either case, damage to the drive screw thread could prevent spin-on of the rivet onto the drive screw. Furthermore, if the thread of the drive screw became worn or damaged whilst the tool were setting the rivet in the workpiece, then the operator might not be able to remove the drive screw from the set rivet—this process is generally known as “spin-off” and involves the drive screw rotating in the opposite sense to that when spin-on occurs. The spin-off process simply removes the drive screw from within the rivet after the setting process is complete.
It will be appreciated that, for efficiency of manufacturing processes, the spin-off operation, after the rivet has been set in the workpiece, should be as rapid as possible so that the operator of the rivet gun to move onto the next rivet which needs to be spun-on to the drive screw and set in a workpiece. Rapid spin-off of the drive screw from the set rivet could be prevented if there were damage caused to the thread of the drive screw.
In order to cater for dealing with damaged drive screws or stripped drive screw threads, for example, the prior art rivet guns, such as that disclosed in EP 0,999,906-A offer the possibility to change the drive screw.
Another reason why there may be the need to change the drive screw could be when a different diameter threaded rivet needs to be installed in a workpiece and the different diameter of the internal rivet thread requires the diameter of the external thread of the rivet gun drive screw to be changed correspondingly. There are such rivet guns known in the art and an example of the retention mechanism for which is shown schematically in FIG. 1. In FIG. 1 a portion of the Avdel® 74201 rivet gun is illustrated. It can be seen that a threaded cap 2 is screwed into a spindle 4 together with a plastic or rubber O-ring 6 which provides an interference fit between the cap 2 and spindle 4 to provide prevailing torque which inhibits vibration loosening. In order to change the drive screw, it is typically necessary to use two spanners—one applied to the flat region 3 of cap 2 and one applied to flat region 5 of the spindle 4 in order to untighten and then re-tighten the two together. This not only takes time, but also requires the correct spanners to be available.
If, on re-tightening, the correct torque has not been applied, it is possible for the cap 2, over time, to unscrew from the spindle 4. This could mean loss of rotational drive and hence prevent spin-on, spin-off or both.